1. Field of the Invention
The subject of the invention is the measurement of the asynchronous vibrations of spinning bodies such as turbine blades by means of sensors placed on the surrounding casing or another fixed position. These measurements are made by sampling, since the sensors can carry them out only when the spinning bodies pass in front of them and are thus called non-intrusive since they are carried out at a distance, without material contact with their object. The sensors used maybe optical, inductive or capacitative. Using stationary sensors is easier than bonding strain gauges onto spinning bodies to measure their distortion directly, which would involve using complex and expensive spinning transmission means or telemetry. Stationary sensors are also able to provide better data than equivalent spinning means, which are subject to difficult and therefore noisy conditions.
2. Discussion of the Background
Using stationary sensors has however the disadvantage of allowing the amplitude of the vibrations to be measured only when the vibrating body moves past, which truncates the measurements of natural frequencies. Indeed a periodic vibratory phenomenon has to be measured at least twice by its highest period to be evaluated without ambiguity, which limits correct measurements to an oscillation frequency band; higher frequency oscillations are measured only with aliasing which makes supernumerary frequencies, with no physical reality, appear in the measurement results.
We are therefore induced to seek a broadening of the band of frequencies that can be detected without ambiguity, in order to reduce the number of natural frequency components located above and subject to aliasing. This can be achieved by reducing the lengths of time between measurements, either by accelerating the rotation of the spinning body, or by increasing the number of sensors assigned to measurement and located over a same circumference so that the vibrating body passes successively in front of them. The first solution is not always practicable and the second has limitations due to the number of sensors to be installed. This is why an improvement of the principle of non-intrusive measurements of vibrations of spinning bodies is hereby proposed to make it easy to distinguish the real vibration frequencies of spinning bodies from the fictitious frequencies, obtained by aliasing natural frequency components above a limit of unambiguous detection, and to remove these fictitious frequencies from the results. The advantage of the improvement is to reduce the number of sensors.
In the improvement proposed as the invention, the detectors are placed in several networks having different numbers of detectors, a spectrum of measured frequencies is obtained for each of the networks, and comparisons of the spectra of the networks are made in order to select spectral lines and to consider as sought vibration frequencies the frequencies of the selected lines. This combination of a new arrangement of the detectors and of an appropriate processing of their results makes it possible to push back appreciably the upper limit of the unambiguous frequency detection band without requiring any very substantial increase in the number of sensors.
Results processing may comprise several phases according to the actual situation to be addressed.